A) Head Injury
Although the brain makes up only 2% of the entire body's weight, it receives 15% of the heart's output of blood and uses up 20% of the oxygen consumed by the body. An organ of this caliber is most vital to survival; within it are control centers for all the senses: sight, smell, touch, hearing, as well as control centers for breathing, hormonal release and all other basic homeostatic functions essential for survival. Damage which renders any portion of the brain dysfunctional can have a devastating effect on an animal's existence, causing neurological and medical problems, and often times death.
Different parts of the brain may be damaged in a wide variety of ways. Common causes of brain injury include vascular diseases and disorders, tumors, infections and actual head trauma.
Vascular disorders can be broken down into three main categories: (1) Problems involving hypoxia, ischemia, and infarction; (2) Intracranial hemorrhage; and (3) Hypertensive cerebrovascular disease.
The brain's dependence on a constant blood supply is of critical importance; it depends on oxygen-rich blood and glucose to function, and the brain is only able to store enough glucose to keep it running for one minute. After four minutes of blood deprivation, irreversible neuronal damage begins. There are two types of acute ischemic injury: (a) ischemic (hypoxic) encephalopathy occurs with a general decrease in cerebral blood flow and causes widespread damage; (b) cerebral infarction occurs following a severe drop or cessation in blood flow to one localized area of the brain. The latter is usually due to a local vascular occlusion and is what many people refer to as "stroke."
Vascular occlusions may be due to clots or arterial plaques but may be due to embolisms (usually from the heart) as well. Intracranial hemorrhage includes intracerebral, subarachnoid, and mixed intracerebral/subarachnoid hemorrhage. Intracerebral hemorrhage usually results from the rupture of aneurysms in hypertensive patients, causing a gradual neurologic deficit such as paralysis, sensory loss, coma or even death. The mortality rate is 40%. Subarachnoid hemorrhage is more superficial and occurs suddenly, usually with physical exertion. 20-50% of these patients die with the first rupture. Mixed intracerebral/subarachnoid hemorrhages are usually associated with arteriovenous malformations (AVM's), which are tangles of abnormal blood vessels both in superficial and deep brain structures. Patients with AVM's often experience seizures.
Hypertensive vascular disease can result in several brain injuries: atherosclerosis, which can lead to "stroke"; lacunae, which are small necrotic areas deep in the brain due to "small vessel stroke;" subcortical leukoencephalopathy, which is diffuse loss of deep white matter due to severe atherosclerosis and loss of perfusion; and finally hypertensive encephalopathy, which is usually seen in malignant (extremely severe) hypertension, and produces headache, drowsiness, vomiting, convulsions, damaged blood vessels, failure of autoregulation of cerebral blood flow, damage of the blood-brain barrier, cerebral edema, and possibly coma.
Brain tumors may either originate in the brain or metastasize to the brain from another part of the body, such as the lung, breast, or intestines. Complications of malignancy include brain degeneration, weakness, tingling and numbness, muscle spasms, dementia, fatigue, confusion, behavioral changes, chemical imbalances, and hemorrhages. Tumors can also impede the blood flow to the brain, resulting in ischemia.
Head trauma is a major cause of the ischemic condition in the brain, as well as causing other damage, such as direct tissue ruptures. In severe trauma, the skull, which is designed to protect the brain, travels faster than the brain on impact, and can actually act as a weapon and cause serious brain damage. In head trauma, the skull, dura and leptomeninges (tissue around the brain), blood-brain barrier (a series of membranes around brain arteries which keep unwanted, harmful molecules out of the brain) and finally the brain itself can all be injured. The four major groups of brain injuries include skull fractures, epidural hematoma, subdural hematoma, and deep brain parenchymal injuries.
Skull fractures can be deadly; the brain can be directly injured by penetrating broken bone fragments. Often, the broken skull causes the rupture of major arteries supplying the brain, usually the middle meningeal artery. Subdural hematomas occur frequently in head trauma due to the rupture of bridging veins in the brain, and usually occur on the side of the brain opposite the impact site. Parenchymal injuries, or injuries to the brain tissue itself, often occur following head trauma. In addition, shearing forces with impact often cause damage to the brian's white matter, a phenomenom referred to as diffuse axonal injury.
B) Chemical Injury
Drugs of abuse and misuse can cause serious brain injury as well. In addition, poisoning by heavy metals in many cases may damage the brain.
Amphetamines have been noted for their ability to cause widespread vascular damage to the brain. Aside from small artery occlusive disease, a condition known as periarteritis nodosa, or necrotizing angiitis, may develop. In this disorder, medium and small arteries of the brain develop aneurysms, sacculatioins, thromboses, and necrosis. In addition, amphetamine abuse has been linked with softening of the cerebral cortex and basal ganglia of the brain, subarachnoid hemorrhages, cerebral edema, ischemia, and infarction.
Cocaine is a potent vasoconstrictor, stimulant and anesthetic. It affects dopamine, norepinephrine, and serotonin neurons in the CNS by blocking the reuptake of these neurotransmitters, ultimately causing their depletion and destruction of dopaminergic pathways in the brain. Pituitary function, which depends on dopamine for some of its regulation, can become deranged, leading to hormonal imbalances throughout the body. Cocaine abusers run a high risk for cerebrovascular disease as well due to excessive vasoconstriction and oxygen depletion.
Nicotine, a component of tobacco, acts as a very potent vasoconstrictor. Carbon monoxide, also ingested by smoking, may impair CNS function by causing an increase in abnormal carboxyhemoglobin and depriving the brain of oxygen. Smokers carry an increased risk of stroke and/or cerebrovascular disease; they are more prone to subarachnoid hemorrhage, hypercoagulable states and cardiac arrhythmias.
Heavy metals can also impair brain function and in some cases lead to the ischemic state. For example, arsenic, commonly found in insecticides, fungicides and herbicides, affects the CNS by blocking aerobic respiration. Clinical symptoms from arsenic poisoning include delirium, coma, and seizures.
C) Neurodegenertive Diseases
A number of neurodegenerative diseases have been diagnosed and studied. Over-excitation of neurons, mediated mostly by glutamate, is believed to be an etiological factor in epilepsy, Alzheimer's Disease, Huntington's Chorea, and cerebral hypoglycemia and ischemia/hypoxia (Foster, A.C., et al, "Protection against N-methyl-D-aspartate receptor mediated neuronal degeneration in rat brain by 7-chlorokynurenate and 3-amino-1-hydroxypyrrolid-2-one, antagonists at the allosteric site for glycine," Eur. J. Neuroscience, 2:270-277(1990)).
D) Functioning of the Brain
In order to understand many of the effects of brain injuries, one must have a general understanding of how the brain operates under normal conditions.
Besides requiring 20% of the body's oxygen supply, the brain also consumes 25% of the body's glucose. The reason for this is the tremendous and constant need for the brain to produce energy. Blood flow to this region must be constant as well due to the brain's inability to store both glucose and oxygen. Under normal circumstances, the brain efficiently converts glucose to energy in the form of ATP by a process dependent on oxygen. The hypoxic brain, in an effort to save itself and still produce energy, relies on the inefficient process of anaerobic glucose metabolism. A by-product of this process is lactate, an acid, which sends a signal to the blood vessels that oxygen is running low. The vessels then dilate in an effort to compensate and supply the brain with more oxygen. If too much lactate accumulates, it impairs the cellular function of the neurons and makes them more susceptible to a second injury; that is, the cell is weakened and unable to handle toxins, chemical imbalances, or slight drops in oxygen which it can normally deal with. As an example, an already traumatized brain may undergo infarction with an ischemic insult that a normal healthy brain could easily tolerate. Furthermore, following severe concussion, there is a surge of adrenaline, which brings along with it an increase in blood glucose levels. Because blood flow is impaired at that time, there is a deficiency of oxygen and the brain reverts to anaerobic glucose metabolism, resulting in a huge surge in lactate, thus weakening the nerve cells.
Because the brain has a constant need for glucose and oxygen-rich blood, the rate of cerebral blood flow (CBF) must be carefully regulated. The brain has developed a delicate system to autoregulate CBF. It is usually remains within the range of 50-60 ml/minute/100 g of brain tissue, and is regulated by a variety of metabolic factors such as stretch of the smooth muscle cells in brain arterioles, changes in cerebral concentrations of oxygen and carbon dioxide (CO.sub.2), blood pH, and nerve responses. Autoregualtion of CBF does not always work, however; when cerebral perfusion pressure exceeds 150 mm Hg and is less than 60 mm Hg, the system fails. After a traumatic brain injury, the sudden compensatory increase in arterial blood pressure exceeds the level at which autoregulation functions; paradoxical reactions and vasospasms begin to occur, clots form, and in general there is a decrease in CBF, hence causing the buildup of toxic metabolic waste products and lack of oxygen.
Another important consideration in brain injury is intracranial pressure (ICP). Two-thirds of patients with severe brain trauma also develop serious increases in ICP. The skull limits the area of space that the brain can occupy; hence traumatic injuries involving brain swelling (edema) or excess bleeding (hematomas) can compress the brain and cut off circulation to certain parts, impairing autoregualtion of CBF, causing permanent damage or even death. Hematomas must be drained as soon as possible to prevent this, and blood pressure must be kept reasonably low to reduce the amount of swelling in contusions.
Complications following brain trauma are often more devastating than the trauma itself. Neurological complications of brain injury include infection, swelling, epilepsy, delayed hemorrhage, amnesia, memory impairment, defects in movement, vision, sensation, and speech, paralysis, and possibly death. Other complications not related to the brain's neurological function include hormonal, cardiovascular, respiratory and gastrointestinal disorders, since control centers for these systems are located in the brain.
E) Treatments for Ischemia and Other CNS Injury
The main objective in the treatment of brain injuries is to minimize any neurological deficits and prevent the progression of further neurological damage. Cerebral ischemia, or stroke, is the most common cause of neurologic disability (Stein & Sabel, PHARMACOLOGICAL APPROACHES TO THE TREATMENT OF BRAIN AND SPINAL CORD INJURY, Plenum Press, New York, 1988). Stroke treatments currently available include calcium channel blockers, anticoagulation and antiplatelet therapy, surgery, and other supportive measures. The following is a description of currently available therapies for brain ischemia caused by strokes.
The principle behind calcium blockade is the finding that local ischemia brings about an increase in intracellular calcium (Ca.sup.+2) (M. Fisher, MEDICAL THERAPY OF ACUTE STROKE, Marcel Dekker, Inc., New York, 1989). The abnormally high Ca.sup.+2 concentration disrupts neuronal membrane pumps, and activates two classes of Ca.sup.+2 dependent protein kinases, which in turn stimulate neurotransmitter release and the hydrolysis of arachidonic acid to prostaglandins and leukotrienes, both vasoactive substances. The excess of excitatory neurotransmitters may lead to cell death; arachidonic acid metabolites aggravate blood flow and stimulate the formation of damaging free radicals. Calcium excess also inhibits cellular respiration (Stein & Sabel, supra).
Calcium channel blockers slow the entry of Ca.sup.+2 into cells. Promising drugs of this class prevent or reverse cerebral vasospasm and dilate cerebral blood vessels, leading to an improvement in cerebral blood flow. The problem with most drugs of this class is the systemic effect of vasodilation in organs other than the brain, which may occasionally end up drawing blood preferentially into these other organs rather than the brain. Other adverse systemic effects include abnormally slowed heart rates and "heart block," the blocking of electrical impulses which traavel through heart tissue; cardiac arrhythmias, congestive heart failure, may occur as well. Dizziness, positional or otherwise, may result from systemic hypotension (PHYSICIAN'S DESK REFERENCE, Medical Economics Co., Inc., Oradell N.J., 1990).
Ca.sup.+2 blockers may also have deleterious effects on ischemic tissue. A study by Welch, et al., found that Ca.sup.+2 blockers may prevent vasospasm in the absence of ischemia, but increase edema formation in the presence of ischemia (Welch, KMA and Barkley, GL, "Biochemistry and Pharmacology of cerebral ischemia, " Stroke 1:75-90, 1986). Some studies in rats have found the well-known Ca.sup.+2 blocking drug Nifedipine to actually antagonize cerebral blood flow (Fisher, supra). Verapamil, another Ca.sup.+2 blocker, has been found in some stroke studies to actually worsen focal ischemia by inappropriately increasing cerebral blood flow to nonischemic areas (Fisher, supra). In a study performed on dogs, Flunarizine, another Ca.sup.+2 blocker, brought about no increase in cerebral blood flow or improvement in cerebral metabolism (Fisher, supra).
Nimodipine is a unique Ca.sup.+2 blocker in that it has been shown in dogs to improve cerebral blood flow with little effect on peripheral vessels and blood pressure. No uniform benefit has been observed, however. One major problem with Nimodipine is that at low doses it acts as a Ca.sup.+2 agonist rather than an antagonist, and actually worsens morbidity. It also may interfere with cellular energy metabolism and increase the susceptibility of tissue to ischemic damage by causing edema and cellular ionic imbalances (Fisher, supra).
Chelating agents, such as EDTA, are occasionally used to bind excess intracellular Ca.sup.+2, but no well-characterized clinical studies of their use in stroke patients have been done (Stein & Sable, supra).
L-glutamate and L-aspartate, both acidic amino acids, act as excitatory neurotransmitters in the mammalian central nervous system. The major glutamate receptor subtype is known as the N-methyl-D-aspartate, or NMDA, receptor (Schoepp, DD, et al, "Neuroprotectant effects of LY 274614, a structurally novel systemically active competitive NMDA receptor antagonist," J. Neural Transmission, 85:131-143, (1991)). Located on the post-synaptic end of the neuron, the NMDA receptor possesses an ion channel as well as multiple regulatory/pharmacological domains, including the transmitter recognition site, to which glutamate and aspartate bind.
Binding of glutamate and other agonists to the NMDA receptor causes excitatory metabolic changes within the cell, including activation of intracellular second messenger proteins which contribute to irreversible neuronal injury, such as protein kinase C, calmodulin, and protein kinase II (Pohorecki, R., et al, "Ischemic brain injury in vitro: protective effects of NMDA receptor antagonists and calmidazolium," Brain Research, 528:133-137, (1990)).
Many researchers have attempted the use of various NMDA receptor antagonists to protect neurons from degeneration in various pathological states. For example, studies have been done on the NMDA competitive antagonist CGS 19755 and the noncompetitive antagonist MK-801 (Warner, M., et al, "Regionally selective effects of NMDA receptor antagonists against ischemic brain damage in the gerbil," J. Cerebral Blood Flow and Metabolism, 11:600-610 (1991)); and the non-competitive NMDA antagonist, dizocilpine (McCulloch, J., "Ischaemic Brain Damage--prevention with competitive and noncompetitive antagonists of N-methyl-D-aspartate receptors," Arznemittel-Forschung, 41:319-324 (1991)); the NMDA antagonist MK-801 (Dirnagl, U. et al, "Pre- and post-treatment with MK-801 but not pretreatment alone reduces neocortical damage after focal cerebral ischemia in the rat," Brain Research, 527:62-68 (19900; Haraldseth, O., et al, "The NMDA antagonist MK-801 improved metabolic recovery after 10 minutes global cerebral ischemia in rats measured with 31 phosphorous magnetic resonance spectroscopy," Acta Neurochirurgica, 106:32-36 (1990).
NMDA receptor antagonists are far from perfect drugs to treat brain injury. Adverse reactions are many and involve many organ systems. MK-801 and CPP have been found to induce respiratory depression and elevated CO.sub.2 level. MK-801 increases blood pressure in rats and cats, while D-CPPene induces hypotension in cats. Noncompetitive antagonists such as MK-801 cause behavioral changes, including a psychotic-like response and diminished cognitive and mental status, even at doses needed for adequate anti-ischemic protection (Meldrum, Brian S., et al, EXCITATORY AMINO ACID ANTAGONISTS, Blackwell Scientific Publications, Oxford, 1991). Other general side effects of NMDA antagonists include central nervous system depression, hallucinations, tolerance development, abuse potential and possible direct neurotoxicity (Turski, L., "N-methyl-D-aspartat-rezeptorkomplex, " Arzenemittel-Forschung, 40:511-519 (1990)).
Preventing and dissolving thrombi, or clots, and maintaining blood viscosity and flow is an additional component of stroke therapy; however, reperfusion can lead to hemorrhage in the area of ischemic or infarcted tissue (Fisher, supra). Fibrinolysis, or the breakdown of already present clots, must be approached with caution; in excess it may actually promote cerebral hemorrhage (Stein & Sabel, supra). Antiplatelet therapy is used to prevent clot formation either postischemically in stroke patients or prophylactically in patients with a history of TIAs. Aspirin, indomethacin, sulfinpyrazone, and ticlopidine have been shown to inhibit platelet aggregation and prevent arachidonic acid from being metabolized into thromboxane A2, a potent platelet aggregator and vasoconstrictor (Zelenock, Gerald B., et al, CLINICAL ISCHEMIC SYNDROMES: MECHANISMS AND CONSEQUENCES OF TISSUE INJURY, The CV Mosby Company, St. Louis, 1990). Platelet anti-aggregants used post-stroke may help to reduce or prevent recurrence and improve microcirculation to ischemically impaired but viable brain tissue (Fisher, supra); however, they are not fast-acting and are not an absolute cure.
Inhibitors of prostaglandin synthesis, such as indomethacin have been found to significantly increase edema and decrease cerebral blood flow and carbon dioxide reactivity in ischemic baboons (Fisher, supra). Administration of prostaglandin I2 (PGI2) has also been attempted in stroke treatment. Unlike most prostaglandins, PGI2 promotes vasodilation and inhibits platelet aggregation. It is far from ideal, though, due to its extremely short half life (3 minutes) and its tendency to precipitate hypotension (Fisher, supra).
Other anticoagulant drugs include heparin and warfarin (Coumadin). These decrease the formation of intravascular thrombosis and embolism and prevent vascular obstruction, but are not widely used in stroke. These drugs carry with them the high risk of brain and/or systemic hemorrhage, and a rebound hypercoagulable state following cessation of warfarin or heparin treatment (Hart RG & Coull, BM, "Hypercoagulability following coumadin withdrawl," American Heart Journal, 106:169-170, 1983; Hart, RG, et al, "Rebound hypercoagulability, " Stroke, 13:527, 1982).
Pentoxifylline, or Trental, improves the oxygenation of ischemic tissues by decreasing blood viscosity by increasing the flexibility of rigid red blood cells, inhibiting platelet aggregation, and decreasing plasma levels of fibrinogen, a substance responsible for clot formation. In addition, it is a mild vasodilator. Many patients with cerebrovascular disease have elevated blood viscosity due to rigid red blood cells and increased tendency toward blood clots. Adverse side effects of Trental can include chest pain, dyspepsia, nausea, vomiting, dizziness, headache, tremor, anxiety, blurred vision, malaise, and others (PHYSICIAN'S DESK REFERENCE, Supra, at 1068-1069).
Another way to increase blood viscosity is by the use of hyperosmolar agents, such as mannitol, sorbitol or glycerol (Stein & Sabel, supra; Zelnock et al, supra). One problem with the use of mannitol and other similar agents is that they usually only provide temporary relief; furthermore, their discontinuation may result in severe rebound cerebral edema (Stein & Sabel, supra). They may disturb cerebral vascular autoregulation as well (Zelenock et al, supra).
Low molecular weight dextran (LMD) has been used for hemodilution as well, and although it decreases platelet aggregation, it exerts a negative effect on plasma viscosity and red blood cell flexibility (L'Etang et al., supra).
Pharmacological protection for stroke victims also involves the use of barbiturates such as pentobarbital and thiopental. Barbiturates decrease the cerebral metabolic rate and intracranial pressure and also may act to scavenge damaging free radicals released by ischemia and block cellular Ca.sup.+2 influx (Zelnock et al, supra). Barbiturates inhibit CNS function by impairing excitatory neurotransmitter release and potentiating presynaptic inhibition by GABA. They must be administered within three hours of the onset of ischemia in order to be effective (Fisher, supra). Their use remains controversial; some animal experiments have found that in order to be effective, extremely high doses are needed (Corkill Get al., "Dose dependency of the post-insult protective effect of pentobarbital in the canine experimental stroke model, " Stroke, 9:10-12, 1978). Complications of barbiturate therapy include the possibility of respiratory depression and cardiac arrhythmias (Ruggieri S, et al, "Barbiturate treatment of acute stroke, " Adv. Neurology, 25:269, 1979). Discontinuation of barbiturate treatment may be followed by a fatal increase in intracranial pressure (Yatsu, FM, et al., "Medical therapy of ischemic strokes, " Stroke, 2:1069-1083, 1986).
Another controversial acute stroke treatment involves the use of naloxone, an opiate (morphine) antagonist. In many clinical studies, its use has been shown to enhance the process of neurological recovery. Due to conflicting results and the fact that most studies have found naloxone to be ineffective in improving neurologic outcome, its use is not widespread. Furthermore, as dosage increases, naloxone tends to produce a harmful increase in systolic blood pressure and respiratory rate (Fisher, supra).
Steroids, such as dexamethasone are occasionally used in stroke treatment as well, to control edema. The use of dexamethasone in vasogenic edema with brain tumors has been successful, but conflicting reports exist as to its efficacy in the cytotoxic edema found in ischemia (Zelenock et al, supra). Many studies on the use of cortisone and dexamethasone for stroke treatment have shown no difference in morbidity or mortality in patients receiving treatment compared with controls (Dyken M, et al., "Evaluation of cortisone in the treatment of cerebral infarction, " JAMA, 162:1531-1534, 1956; Candelise L, et al . , "Therapy against brain swelling in stroke patients," Stroke, 6:353-356, 1975; Gilsanz V, et al., "Controlled trial of glycerol versus dexamethasone in the treatment of cerebral edema in acute cerebral infarction, " Lancet, 1:1049-1051, 1975). Furthermore, dexamethasone can cause serious exacerbations of diabetes mellitus (Norris JW, "Steroid therapy in acute cerebral edema," Archives of Neurology, 33:69-71, 1976). Perhaps most serious is the observation of Sapolsky & Pulsinelli, who in 1985 noted that ischemic injury was actually potentiated by steroids, perhaps via the release of excitatory neurotransmitters, leading to ATP depletion and an accumulation of intracellular Ca.sup.+2 (Fisher, supra).
The neurotransmitter dopamine may be involved in stroke injury and recovery. Experimental studies have found dopamine blockade to inhibit recovery from strokes, especially in older animals. Another neurotransmitter which may be involved with stroke pathology is serotonin. Experiments have shown serotonin antagonists such as cinanserin and cyproheptadine to help preserve neurologic function when administered post-ischemia in animals. Tissue destruction and clinical deficits are diminished. The reason for this observed phenomenon is unclear, and this type of therapy is not widely used (Stein & Sabel, supra).
Surgical treatment of stroke most recently involves the use of percutaneous transluminal angioplasty (PCTA). PCTA has proven relatively successful; it is not perfect, however, and cannot be used immediately on a patient unless that patient is already hospitalized. Complications can include vessel dissection and occlusion if the vessel is overdistended by the balloon. Compression and occlusion of adjacent vessels may occur as well, due to balloons that are too long. Embolism may occur as well. Problems specific to carotid artery PCTA are linked to the fact that the carotid is especially inclined to spasm.
Recently, work using diazepam has shown it to be somewhat effective in treatment of cerebral ischemia. (R.A. Huff et al., "Diazepam following cerebral ischemia preserves CA1 pyramidal cells and GABA.sub.A receptors of the hippocampus," Soc. for Neuroscience Abstracts, 7:1079 (1991); C. L. Voll et al., "Postischemic seizures and necrotizing ischemic brain damage: neuroprotective effect of postischemic diazepam and insulin," Neurology 41:423-428 (1991)).
F) Peripheral-Type Benzodiazpine Receptors
Benzodiazepines (BZs) are a family of compounds which include drugs used as tranquilizers, sleeping aids, muscle relaxants, and anti-convulsants, and include Valium, Halcion, and Xanax. In 1977, specific BZ receptors were discovered in the neuronal cell membranes in the brain. S. H. Snyder, DRUGS AND THE BRAIN (1986) Scientific American Books, Inc., pp. 167-169. At that time, no naturally occurring agonists or antagonists of the BZ receptors were known. Although the question has not yet been completely answered, several discoveries involving the BZ receptor and its function have been made since 1977.
One major finding was that GABA, an inhibitory neurotransmitter which slows the neuron's rate of firing, stimulated the binding of BZs to the BZ receptor, and likewise, BZs stimulated the binding of GABA to its receptor. Each enhanced the neuronal inhibitory action of the other, by increasing the ease of opening of the neuron's Chloride (Cl) channel and increasing Cl conductance, thus decreasing the activity of the nerve cell. It has now been shown that there exist receptors, originally found in the brain and therefore termed "central-type" BZ receptors, which are associated with a receptor complex which has binding sites for GABA.sub.A, BZs, barbiturates and convulsants. This complex is called the GABA.sub.A receptor-chloride ionophore complex (GRC), and is located near the neuron's Cl channel. When both GABA and BZ bind to their respective receptors, the Cl channel opens with greater frequency, thus allowing for a great influx of Cl into the cell and diminished neuronal activity. Convulsants such as picrotoxin cause the opposite effect. S. H. Snyder, supra, pp. 174-176.
In 1977, researchers found that BZ receptors were present not only in the brain and spinal cord (CNS), but in other organs, such as the heart, as well. W. E. Muller, THE BENZODIAZEPINE RECEPTOR: DRUG ACCEPTOR ONLY OR A PHYSIOLOGICALLY RELEVANT PART OF OUR CENTRAL NERVOUS SYSTEM (1987), Cambridge University Press, pp. 32-33. These receptors have a different substrate specificity; i.e., they bind some BZ derivatives with a different affinity than do the central-type receptors, and are believed to modulate a calcium (Ca.sup.+2) channel as well. In addition to binding BZ derivatives, these receptors have been found to bind various organic compounds such as isoquinoline carboxamides, quinoline propanamides, imidazopyridines, porphyrins, thiazide diuretics, and others. Krueger, et al., "Purification, Cloning, and Expression of a Peripheral-Type Benzodiazepine Receptor," in Biggio and Costa, eds., GABA AND BENZODIAZEPINE RECEPTOR SUBTYPES,pp. 1-14 (1990).
These so-called "peripheral-type" BZ receptors have also been found in the CNS as well, but with a different distribution than the central-type BZ receptors. W. E. Muller, supra, pp. 33, 75. Both types are found in high density in the olfactory bulb, but unlike central-type BZ receptors, peripheral-type BZ receptors also are concentrated in the pineal gland, posterior pituitary, and choroid plexus. Furthermore, patients with Alzheimer's Disease have been found to have an elevated amount of peripheral-type BZ receptors in the cortex of their frontal lobes, and patients suffering from Huntington's Chorea have an abnormally high concentration of these receptors in the putamen, compared to patients without these diseases. W. E. Muller, supra at 75-76; D. Diorio et al., "Peripheral benzodiazepine binding sites in Alzheimer's disease frontal and temporal cortex," Neurobiol. Aging, 12:255-258 (1991).
The peripheral-type BZ receptors have been cloned in order to allow further study of the pharmacological actions of BZs and other molecules on these receptors. Krueger, et al., supra.
Recently, evidence has developed which suggests that the number of peripheral-type BZ receptors is directly related to neurological trauma, and may be used as a marker in rodent brains for excitotoxic, ischemic and proliferative damage. Benavides et al., "The quantification of brain lesions with an .omega..sub.3 site ligand: a critical analysis of animal models of cerebral ischaemia and neurodegeneration, " Brain Res. 522:275-289 (1990). Additionally, it has been demonstrated that the number of peripheral-type BZ receptor sites increases in the brain when certain cytokines are administered locally. F. Bourdiol, et al., "Increase in .omega..sub.3 (peripheral type BZ) binding sites in the rat cortex and striatum after local injection of interleukin-1, tumor necrosis factor-.alpha. and lipopolysaccharide," Brain Research 543:194-200 (1991). The data produced by these researchers suggests that the increased number of these peripheral-type receptors may be due to increased numbers of glial cells at the injection site.
Other recent work has shown that the binding of BZs at peripheral-type sites inhibits proliferation of a number of cell types. J. K. T. Wang et al., "Benzodiazepines that bind at peripheral sites inhibit cell proliferation," Proc. Natl. Acad. Sci. (USA), 81:753-756 (1991).
Several theories have been advanced for treatment of brain trauma through the administration of various compounds. Several groups have suggested that diazepam acts to prevent damage to the brain by decreasing excitatory neurotransmission at the GABA.sub.A receptor-chloride ionophore complex (GRC). C. L. Voll et al., supra; R. A. Huff et al., spra. However, no evidence has been produced to show that BZs which bind solely to the excitation-modulating central-type BZ receptors can have any positive effect on cerebral ischemia.
However, studies done on compounds other than diazepam which bind to the peripheral-type BZ receptors have found them to be ineffective in treating cardiac ischemia. G. Drobinski, et al., "Absence d'effet anti-isch emique d'un antagoniste des r ecepteurs p eriph eriques aux benzodiazepines PK 11195, " Therapie, 44:263-267 (1989).
Another theory advanced is that the treatment of head injury is through the prevention of excess oxidation at the site of injury. Jacobsen, E. J., et al., "Novel 21-aminosteroids that inhibit iron-dependent lipid peroxidation and protect against central nervous system trauma," J. Med. Chem., 33:1145-1151 (1990). However, the active compounds of the present invention do not act in this manner.